The invention is an improved golf club head. More particularly, the invention is a golf club head with an improved striking face having discrete zones with varying flexural stiffness, improved accuracy, and a high coefficient of restitution.
The complexities of golf club design are well-known. The choice of specifications for each component of the club (i.e., the club head, shaft, hosel, grip, and subcomponents thereof) directly impacts the performance of the club. Thus, by varying the design specifications, a golf club can be tailored to have desirable performance characteristics.
The design of club heads has long been studied. Among the more prominent considerations in club head design are loft, lie, face angle, horizontal face bulge, vertical face roll, face progression, face size, sole curvature, center of gravity, material selection, and overall head weight. While this basic set of criteria is generally the focus of golf club engineering, several other design aspects must also be addressed. The interior design of the club head may be tailored to achieve particular characteristics, such as by including hosel or shaft attachment means, perimeter weighting on the face or body of the club head, and fillers within hollow club heads.
The designs for golf club heads must also be strong enough to withstand the impact forces that occur during collision between the head and the ball. The loading that occurs during this transient event can cause an acceleration to the golf ball that is four orders of magnitude greater than that of gravity. Thus, the club face and body should be designed to resist permanent deformation or catastrophic failure, by material yield or fracture.
It is not unusual for club heads of prior art hollow metal woods, produced from titanium, to have a uniform face thickness exceeding 0.15 inch. This thickness has been required to ensure structural integrity of the club head during impact.
Players generally seek a wood and golf ball combination that delivers maximum distance and landing accuracy. The distance a ball travels after impact is dictated by the magnitude and direction of the ball""s translational velocity and the magnitude and direction of the ball""s rotational velocity or spin. Environmental conditions, including atmospheric pressure, humidity, temperature, and wind speed further influence ball flight. However, these environmental effects are beyond the control of the golf equipment manufacturer. Golf ball landing accuracy is driven by a number of factors as well. Some of these can be attributed to club head design. Primarily, of concern here are center of gravity and club face flexibility.
Golf ball distance is a function of the total kinetic energy imparted to the ball during impact with the club head, neglecting environmental effects. During impact, kinetic energy is transferred from the club and stored as elastic strain energy in the club head and the ball. After impact, the stored elastic energy is transformed back into kinetic energy in the form of translational and rotational velocity of the ball as well as the club. Since the collision is not perfectly elastic, a portion of energy is dissipated in club head vibration and viscoelastic relaxation of the ball. Viscoelastic relaxation is a material property of the polymeric materials used in all manufactured golf balls.
The United States Golf Association (USGA), the governing body for the rules of golf in the United States, has specifications for the performance of golf balls. These performance specifications dictate the size and weight of a golf ball that conforms to the USGA. Furthermore, there are USGA rules which limit the golf ball velocity after a prescribed impact to 250 feet per second xc2x15%. To achieve greater golf ball distance, ball velocity after impact must be maximized while remaining within these guidelines. Viscoelastic relaxation of the ball is a parasitic energy source, which is dependent upon the rate of deformation. To minimize this effect, the rate of deformation must reduced. This may be accomplished by allowing more club face deformation during impact. Since metallic deformation maybe purely elastic, the strain energy stored in the club face is returned to the ball after impact thereby increasing the ball""s outbound velocity after impact.
A variety of techniques may be utilized to vary the allowable deformation of the club face. For example, uniform face thinning, thinned faces with ribbed stiffeners and a varied thickness on the face profile are three possibilities. Any design must have sufficient structural integrity to withstand impact without permanent deformation of the club face.
In general, prior art club heads exhibit large variations in the coefficient of restitution (COR) magnitude with impact location on the face of the club. Furthermore, accuracy of those prior art clubs are highly dependent on this impact location.
Thus, there is a need for a golf club with a face that maximizes golf ball distance while maintaining accuracy due to a reduced sensitivity to face impact location. Furthermore, it would be desirable for the improved club design to minimize the dissipation of spurious energy modes of structural vibration of the club to further maximize efficient energy transfer after impact.
The present invention relates to a golf club head adapted for attachment to a shaft. The head includes a face and a body. The face is configured and dimensioned so that it includes a central portion and an adjacent surrounding intermediate portion. The central portion is rigid and the intermediate portion is relatively flexible so that upon ball impact, the intermediate portion of the face deforms to provide high ball velocity, while the central portion is substantially undeformed so that the ball flies on-target. Thus, upon ball impact the deformation of the intermediate portion allows the central region to move into and out of the club head as a unit. As a result, the head exhibits a coefficient of restitution greater than 0.81.
The above is accomplished by providing the central portion with a first flexural stiffness and the intermediate portion with a second flexural stiffness. Flexural stiffness is defined as the portion""s Elastic modulus (E) times the portion""s thickness (t) cubed or Et3. The first flexural stiffness is substantially different from the second flexural stiffness. As a result, upon ball impact, the intermediate portion exhibits substantial deformation so that the central portion moves into the club head, and the central portion exhibits minimal deformation.
In one embodiment, the first flexural stiffness is at least three times the second flexural stiffness. In other embodiments, the first flexural stiffness is six to twelve times the second flexural stiffness.
More preferably, the first flexural stiffness is greater than 25,000 lb-in. Most preferably, the first flexural stiffness is greater than 55,000 lb-in. Preferably, the second flexural stiffness is less than 16,000 lb-in. More preferably, the second flexural stiffness is less than 10,000 lb-in.
Since the flexural stiffness is a function of material and thickness, the following techniques can be used to achieve the substantial difference between the first and second flexural stiffnesses: 1) different materials can be used for each portion, 2) different thicnkesses can be used for each portion, or 3) different materials and thicknesses can be used for each portion. For example, in one embodiment, the thickness of the central portion is greater than the thickness of the intermediate portion and the material for both portions is titanium.
The golf club head further includes a perimeter portion disposed between the intermediate portion and the body. In one embodiment, the perimeter portion has a third flexural stiffness that is at least two times greater than the second flexural stiffness. The area of the perimeter portion preferably comprises less than 30% of the total area of the club head face.
In an alternative embodiment, a golf club head includes a shell that defines an inner cavity and a face. The face defines a face area and includes a first portion in the center and a second portion adjacent thereto. The first portion has a first thickness and defines a first area. The second portion has a second thickness. The first area is between about 15% and about 60% of the total face area, and the first thickness is greater than the second thickness. More preferably, the first area is between about 20% and 50% of the face area.
In the club heads discussed above, the first, second, and third portions can have various shapes, such as the shape of the face or an elliptical shape. Furthermore, the club head inner cavities can have a volume greater than about 250 cubic centimeters, and more preferably a volume greater than about 275 cubic centimeters. It is recommended that the face of the club head have a loft of greater than about 13xc2x0.
In addition, the central, intermediate, and perimeter portions can each have variable thicknesses. One feature of the present invention is specifically locating the center of gravity of the club head with respect to a first, second and third axes. The shell further includes a top portion and a spaced sole plate, a heel portion and a spaced toe portion, and a rear spaced from the face. The first axis extends between the top portion and the sole plate. The second axis extends between the heel portion and the toe portion. The third axis extends between the face and the rear portion. The axes meet at the geometric center of the face and the center of gravity is defined with respect to the geometric center of the face. The center of gravity is preferably toward the middle of the second axis and on the third axis at or below the first axis such that the center of gravity is behind the center of the face or lower. Preferably, the center of gravity is on a point of the third axis within the central portion. In one embodiment, the center of gravity is spaced from the geometric center along the first axis by a first distance of at least about 0.1xe2x80x3. More preferably, the center of gravity is spaced from the geometric center along the first axis toward the sole plate, wherein the first distance is at least about 0.15xe2x80x3. In another embodiment, the center of gravity is spaced a second distance from the geometric center along the second axis, wherein the second distance is less than about 0.02xe2x80x3. In addition, the center of gravity is spaced a third distance from the geometric center along the third axis toward the rear portion, wherein the third distance is less than about 1.25xe2x80x3.
The present invention is also directed to a golf club head adapted for attachment to a shaft that comprises a face. The face includes a total face area and first primary resonant frequency, which is less than about 2900 Hz. The face further includes a central zone that includes a geometric center of the face, and an intermediate zone disposed adjacent the central zone. The central zone has a first flexural stiffness and a central zone area that is at least 15% of the total face area. The intermediate zone has a second flexural stiffness. The first flexural stiffness is at least 25,000 lb-in and greater than the second flexural stiffness.